Touch-sensitive switch employing electret foil

ABSTRACT

A keyboard switching arrangement that is mechanically simple, reliable, and which can be fabricated with an overall thickness of less than 1/4 inch, includes a plurality of independent conductive elements aligned with holes in a protective front cover. A single sheet of a metallized foil electret is interposed between the cover and the conductive members. The metal layer of the foil forms a capacitive transducer with each backplate section. Each switch is operated by touching the foil electret through one of the cover holes. The resulting displacement generates an electric output signal which is utilized for actuating an associated switching element or the like.

United States Patent Sessler et al.

[54] TOUCH-SENSITIVE SWITCH EMPLOYING ELECTRET FOIL [72] lnventors: Gerhard Martin Sessler, Summit; Robert Lee Wallace, Jr., Warren Township, Somerset County; James Edward West,

Union, all of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: Dec. 28, 1970 [21] Appl. No.: 101,536

[4 1 June 6, 1972 Primary Examiner-Robert K. Schaefer Assistant Examiner-William J. Smith Attomey-R. J. Guenther and William L. Keefauver 1 ABSTRACT A keyboard switching arrangement that is mechanically simple, reliable, and which can be fabricated with an overall thickness of less than /4 inch, includes a plurality of independent conductive elements aligned with holes in a protective front cover. A single sheet of a metallized foil electret is interposed between the cover and the conductive members. The metal layer of the foil forms a capacitive transducer with each backplate section. Each switch is operated by touching the foil electret through one of the cover holes. The resulting displacement generates an electric output signal which is utilized for actuating an associated switching element or the like.

26 Clairm, 5 Drawing Figures PATENTEDJuu e 1912 SHEET 10F 2 4% ATTOR EV G. M. SESSLER lNl WTO R. L. WALLACE, JR.

J E 227T UV] ADD E R FIG J 20 2 TOUCH-SENSITIVE SWITCH EMPLOYING ELEC'IRE'I FOIL This invention relates to a signalling apparatus and more particularly to key selector apparatus that is sensitive to the touch. It has for its principal object the improvement of touch selectors through a simplification of their mechanical construction, the elimination of metallic contacts, and an increase in reliability. Other objects are a reduction of size, an improvement in sensitivity, the reduction of cost, and the generation of an output signal that may be used without additional detection or processing.

BACKGROUND OF INVENTION Numerous signalling operations require the generation of voltage pulses or tone signals according to a prescribed code. For example, the conventional telephone instrument employs a dial arrangement which actuates metallic contacts to interrupt a source of current-in the production of sequences of binary pulses. Pushbutton selectors used on modern telephones mechanically establish circuits which initiate the generation of multitone signals. Pushbutton controllers have gained an additional degree of importance as input devices for electronic calculato'rs and the like.

As with all mechanical switching arrangements, however, ohmic contacts occasionally become contaminated and fail. More importantly, mechanical arrangements are relatively heavy and bulky, exhibit high inertia, and because of the precision required for their fabrication, contribute significantly to the total cost of the signalling arrangement.

DESCRIPTION OF THE PRIOR ART To avoid the difficulties inherent in mechanical dial or button selectors, a variety of alternative switching arrangements, such as crosspoint matrices and magnetic and electrostatic units, have been developed. For example, the spring element of an actuator may be replaced by a single sheet of pliable material overlaying a matrix array of conductors. As the pliable cover sheet is depressed in a defined location, a corresponding ohmic contact is established as the underlying conductors are forced into contact. Similarly, signalling apparatus employing touch-sensitive electrostatic elements, represented for example by Learner, U.S. Pat. No. 3,281,541, avoid many of the difficulties of mechanical units.

In a typical electrostatic unit, a change in capacity between a touchplate and a reference electrode, occasioned by the placement of a finger on the plate, is detected and used to generate a useful signal. Although such electrostatic and film membrane units are compact, they nevertheless require auxiliary energizing power and detector and signal generating equipment to prepare suitable switching signals.

SUMMARY OF THE INVENTION The shortcomings of the prior art are overcome in accordance with this invention by capitalizing on the unique proper-ties of a charged thin film membrane. Such a charged film is known as an "electret and, when the film is provided with a thin conductive coating on one of its surfaces, it is generally known as a metallized foil electret or simply as a metallized electret." Electrets exhibit a permanent charge and, when employed in a transducer-like configuration, develop sizable output signals when displaced.

In a typical touch signalling arrangement in accordance with the invention, a plurality of separated conductive elements are supported in any desired pattern in an insulating member. All of the elements are covered by a thin metallized electret sheet, or a plurality of such sheets, held a slight distance away from the elements and from each other. A front cover, equipped, for example, with apertures of finger size, is supported in alignment with the metallic conductive elements. As the foil is mechanically displaced in relation to one of the conductive elements, a signal pulse is produced in a connecting output circuit. No auxiliary detector is required. Ac-

cordingly, the signal is sufficient for actuating auxiliary oscillators, switching elements, amplifiers, or the like.

The touch selector, in accordance with the invention, may be modified in a number of ways to adapt it to a particular application. For example, an auxiliary, uncharged, film may be placed between the metallized foil electret and the aperture cover to protect the electret against environmental, manual, or other hazards. Moreover, pushbuttons may be employed as an interface between the user and the electret foil. If desired, several metallized electret sheets may be sandwiched between the aperture plate or protective membrane and the array of conductive backplate elements to provide trains of pulses of positive and negative polarity. Such pulse trains may be used to address the logic circuits of connecting devices.

Regardless of the arrangement of elements employed, the electret touch selector, in accordance with the invention, is extremely compact, simple to implement, and reliable. In a typical unit constructed for use as the touch selector for a telephone instrument in place of the usual dial selector, an output signal of more than 8 volts into a load 10'' ohms was produced by a unit having an overall thickness of less than It inch in response to an applied force in the neighborhood of 5 X 10 dynes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be fully apprehended from the following detailed description of preferred illustrative embodiments thereof taken in connection with the appended drawings.

In the drawings:

FIG. 1 is an exploded view of the basic elements of a touchsensitive selector in accordance with the invention;

FIG. 2 illustrates, in cross section, the basic arrangement of a call selector in accordance with the invention, which employs an auxiliary protective film and pushbutton guide members;

FIG. 3 illustrates an arrangement in accordance with the invention in which a multiplicity of individual metallized foil electrets are employed to produce coded pulse trains of two polarities;

FIG. 4 illustrates a trapezoidal displacement of the foil of an electret selector in accordance with the invention and the voltage response of the unit for two different time constants; and

FIG. 5 illustrates the voltage of one unit of an electret touch-sensitive selector in accordance with the invention as a function of the applied force for various terminating resistances.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates, by means of an illustrative exploded view, the major elements of a touch-sensitive selector switch in accordance with the invention. The selector employs a plurality of conductive backplates 11,, 11,, embedded in an insulating support member 12. Preferably backplates l 1 are recessed a slight distance beneath the front surface of support member 12. In a typical arrangement employed in practice, each of the backplate members 11 is approximately 1.6 cm. in diameter and the support 12 is composed of a plastic insulating material. The backplates in this embodiment are recessed by approximately 50 am. into the insulator.

If desired, the individual backplate members may be conductive areas printed on a circuit board. If the conductive portions are on the surface of the board, an apertured insulating layer is used to establish the required recess. Otherwise, the conductive elements may be deposited in recesses in the board.

A sheet of a metallized electret material 13 is positioned over the backplates and rests generally on the surface of support 12. In one example of practice, an electret foil was prepared from a single 2 mil (51 pm.) thin film of polyfluoroethylenepropylene plastic material, marketed commercially as Teflon FEP, with a 1,000 A. metallic layer on mm ul its surfaces. When polarized, such an electret full typically enltlltits a t'harge nl' about 10' (.oulombtt per square centimeter.

Metallized electret 13 is held in place by a thin cover 14 having a plurality of holes 15 1 m aligned with the conductive backplates ll,,, 11,,, in support 12. In this configuration, the metallic layer on film 13 faces aperture plate 14. If desired, plate 14 may be formed of conductive material to provide additional electrical shielding for the unit. The coverplate is secured for example to a conductive case 16 which encloses insulating member 12. Each of conductive plates 11 is connected to one of terminals 17 17 which extend through the unit and are exposed for external connections. The other electrical connection (not shown) is made to metallized coating 13c on foil 13. If coverplate 14 is conductive, and if no auxiliary layers are used, connection may be made to plate 14. Obviously, other mounting techniques may be employed.

The selector mechanism is operated by touching one of the exposed foil sections briefly through one of apertures 15 in coverplate 14. As the foil is touched, it is displaced nonuniformly over the area of the aligned backplate 11. After the foil is released it returns to its quiescent position due to its tension and stiffness. As a result of these operations, the foil electret 13 and the juxtaposed backplate act as an electromechanical transducer, as opposed to an electroacoustic transducer, and generate a voltage pulse v(t) across an impedance connecting the metallic surface 130 of electret 13 to backplate terminal 17.

The generated voltage pulse may be described by either of the convolutions (2) where s(t) is the (spatial) average displacement of the foil electret, .i-(t) is the time derivative of s(t), and h(t) and a(t) are the unit-impulse and unit-step responses to displacement, respectively. The selector can thus be visualized as a circuit which includes a series connection of the capacitance Got one of the electret transducers (foil 13 and conductive element 11), a terminating resistor R (not shown), and a voltage source responsible for the voltage generated by the electret transducer. This circuit is characterized by a(t) Ae' for re where 8(1) is the Dirac delta function and A is the ratio of voltage generated by the voltage source to average displacement of the foil electret. Thus, A is given by A a'D/(D+ed),,. s

In equation o is the effective surface charge density of the electret on its nonmetallized surface, D and d are the thicknesses of electret and air layer between electret 13 and backplate 11, respectively, 6 is the dielectric constant of the electret material and s the permittivity of free space. Equation (5) holds if the displacement s(!) is much smaller than the spacing d.

As an example, the voltage v(t) for the trapezoidal displacement function shown in FIG. 4a is given by where At t t, is the risetime of the displacement function, and s the maximum (spatially averaged) displacement.

Utilizing equations (2) and (4), the voltage v(t) across R is and substituting the resulting v(t)At ASORC- 0 for t :2

The function v(t)/As,, is plotted for RC 0.1 t, and RC 10 2 in FIGS. 4b and 40, respectively. The assumption that t 0.9 1 is made in both cases. The figure shows that for RC r theresponse of the system is a distorted (-difierentiated") replica of the displacement consisting of two spikes, while for RC a response resembling the displacement function is obtained.

To summarize, for a trapezoidal displacement of a metalized electret, the electrical output signal across a terminating resistor consists of a pair of oppositely poled spikes with durations comparable to the time constant of the system if the time constant is smaller than the duration of the displacement. For time constants greater than this, the electrical output approximates the displacement. Since the output voltage of the unit is dependent upon the product of the field in the air gap between the foil and the backplate and the displacement of the foil into the gap, the extent of the air gap, as determined for example by the depth of the recess in support member 12, should be selected in accordance with foil parameters and expected displacement. If the recess is too large, the field is small; if the recess is too small, displacement is restricted. It has been found that a recess depth on the order of the foil thickness is satisfactory.

For an electret with a charge density of 10* C/cm.", pulse amplitudes due to touching have been measured to be on the order of 0.3 to over 20 volts depending on the value of the terminating resistance and the applied force. Output voltage developed in a typical unit, as a function of applied force, is illustrated in FIG. 5. Although the output voltage rises with force for small force levels, it becomes independent of force at larger levels. This is because At, the rise time of the displacement function, becomes much smaller than the time constant of the system as the force level is increased. Equation (8) shows that for At much smaller than RC, the amplitude of the generated voltage is essentially independent of At.

Signal voltages of this sort can be used directly without further detection and often without additional processing. For example, the produced signal may be coupled directly into an amplifier circuit having a reasonably high input impedance. A single-stage, field-effect transistor amplifier has been found to be a suitable input circuit element. Alternatively, output signals may be amplified in any bi-polar transistor amplifier, for example, one using multiple stages and negative feedback to achieve the required input impedance.

An alternative configuration which may be used in the fabrication of a selector switch in accordance with the invention, is illustrated schematically in FIG. 2. Here, the construction shown in cross section, is much the same as that of the apparatus in FIG. 1 except that an auxiliary, uncharged, thin pliable film 18 is interposed between foil electret 13 and apertured coverplate 14. It is often desirable to use such an auxiliary layer to protect the foil electret from environmental or other damage, and to assure maintenance-free operation. Film 18 preferably is bonded to case 16 (FIG. 1) to provide maximum protection for the selector unit.

It is also sometimes desirable to employ formed pushbuttons or the like to facilitate operation of the unit. Since pushbuttons are in common use, they give the unit a more conventional appearance. More importantly, pushbuttons or other mechanical actuators serve to assure more uniform displacement of the electret means. In the drawing, buttons 19 are formed with a concave surface configuration and are held captive in the apertures of plate 14. The button shape and size is a mere matter of design choice. When depressed, they cause the necessary displacements of electret 13 with respect to backplate member 11 to generate an output signal. By properly proportioning the length of the button shaft extending through the apertures, a stop-action is assured to prevent possible physical damage to the foils. Although the pliable films are generally resilient enough to return the buttons to their withdrawn position, it may be desirable to employ auxiliary spring elements.

If desired, auxiliary means (not shown) may be employed to 1 provide an indication that contact has been achieved. For example, the finger aperture or button element may be illuminated as contact is made, or an audible or other visual indication may be provided to notify the user of a completed contact. Such indicating elements are well known to those skilled in the art and include such things as buzzers, cricket devices, and analog or digital display-windows. Conveniently, electrical contact between a portion of a pushbutton, e.g., 19, and printed circuit connectors on auxiliary layer 18 or in insulated channels associated with cover 14 may be used.

As a further aid in establishing a desired switching sequence, suitable identification marks may be used in association with each of the touch selector positions. For example, suitable position indications may be printed or individually attached to the outward surface of the foil in each aperture position. If auxiliary protective film is employed, such indications preferably are placed beneath the protective layer. If button elements are used, the position designation preferably is placed in the button housing in the usual fashion.

In accordance with yet another embodiment of the invention, shown schematically in FIG. 3, multiple foils of electret material are employed to generate a train of either positive or negative polarity pulses for each displacement. Such pulse trains are generated by employing a plurality of metallized foil electrets. In the figure, two foils 13 and 20, are placed across a single backplate member 11, with both metallic layers 13c and 20c facing away from the electrode. A slight air gap 21 is maintained between the foil layers, for example, by insulating spacers 22. Spacers 22 may be in the form of discrete insulators, a grid configuration, or an insulating aperture plate aligned with backplate members 11. Electrical connections are made between the conductive surfaces 13c and 20c of foils l3 and 20 and terminal 17. Preferably, the conductive layer of the outer foil, e.g., 200 is maintained at ground potential to provide electrostatic shielding for the unit. The several signals from the foils are delivered directly to adder 23 and the signal from electrode 11 developed on terminal 17 is passed through delay element 24 and thence to adder 23. The interval of delay of unit 24, d, is selected to provide the desired spacing between the pulses of the resulting train.

With this multiple layer configuration, two or more capacitive switching elements are in effect established in series. Thus, electret foil 20 acts together with the conductive layer 130 of foil 13 as one switching element and electret 13 in cooperation with backplate 11 constitutes the other. As the foils are depressed toward backplate 11, an initial signal output is developed between electret foil 20 and conductive layer 13c which acts as its associative backplate. This signal, delivered by way of adder 23, constitutes the first pulse in the train. As the foils are further depressed, a second signal is generated as foil 13 is moved in relation to backplate 11. This signal is delayed in unit 24 so that it occurs at a time d following the generation of the first pulse. In this example, it is assumed that both foils are of like polarity to give rise to two positive pulses. If the foils are both negatively or both positive- 1y polarized, spaced apart pulses of like polarity are produced. If one foil is positive and the other negative, pulses of opposite polarity are produced. Alternatively, delay unit 24 may be placed, for example, in the conductor interconnecting foil 20 with adder 23. By this expedient, the signal developed between foil 13 and backplate 11 can be made to appear in advance of the pulse produced between foil 20 and the conductive layer of foil 13 in the pulse train. Obviously, by selecting the polarities of the foil electret layers and the position and number of delay units employed, any desired pulse train configuration may be produced. If desired, the output signal developed between one foil electret and the conductive element of another electret may be delivered to an output circuit independent of the output circuit which receives the signal derived at the same touch location between a foil electret and the corresponding conductive backplate element.

Moreover, since an electret foil may be polarized with opposite polarities at discrete lateral locations, the portion of the electret foil aligned with each touch location may be either positively or negatively polarized. Accordingly, the signal developed at each touch location is coded according to polarity. Further, if multiple foil electrets are used, with or without a backplate element, similar polarity coding may be used at each touch location. Hence, signals derived from different electret layers at a single touch location, being of different polarity, may be delivered to separate utilization circuits, independent of the circuits used to accept delayed signals from that touch location.

Additional flexibility is obtained by restricting the metallized layer on selected electret foils to discrete areas in the vicinity of each touch location, and by independently connecting the metallized areas of those foils to external circuits. Known techniques may be used to establish such areas. Regardless of the way in which the signals are derived or combined as a composite signal, they are typically delivered to a utilization device such as amplifier 25, or the like.

Although the invention has been described with reference to a number of specific embodiments, it will be evident to those skilled in the art that numerous combinations of the features described above may be used as required. Thus, for example, the aperture arrangement of FIG. 1 may be implemented with a plurality of electret films, as shown in FIG. 3, and may additionally employ a protective film of the sort shown in FIG. 2. Similarly, in the button arrangement of FIG. 2, a single electret foil may be used as opposed to the foil and protective film arrangement illustrated. Furthermore,'multiple electrets may be used in this configuration. Suffice it to say, the arrangements illustrated are merely representative of numerous alternatives.

As evident from the detailed description of various embodiments of the touch selector of the invention described above, construction is exceedingly simple and no critical dimensions are involved. Moreover, with modern film technology the entire unit may be made extremely thin so that it may be used in a variety of applications. Obviously, single selector units, i.e.,

an arrangement using one touch point with one backplate may be used as required for single indicator applications. Thus, there is no need to employ an entire array of touch points in accordance with the invention, although great economies result from so doing. Moreover, the touch points may be arrayed in any desired configuration, e.g., circular, in-line, or the like.

In practice, the touch sensitive selector described herein has not exhibited any adverse effect as a result of frequenttouching. In experimental use, the output voltage of a unit similar to that shown in FIG. 1 did not change measurably after more than 400,000 automatic touch operations performed with a motor-driven plunger directly contacting the metallized layer of the electret foil. Even so, it is desirable, as illustrated in FIG. 2, to use an additional plastic film for protection against hazards of manual use. Moreover, sensitivity of the selector elements to airborne sound is comparatively small. It was found that a pistol shot fired at a distance of 3 feet generated a signal more than 70 db. less than that resulting from a touch operation. Thus, to actuate the selector, a mechanical (tactile) force, not an acoustic one, must be applied. One must touch the selector to operate it, not yell at it. When the switch arrangement was dropped from an elevation of three feet, the resulting signal was from 30-50 db. less than that resulting from touching. In a configuration dimensioned for convenient finger operation, crosstalk generated at neighboring units typically is 40 db. less than the signal generated by touching the unit.

Numerous alternative arrangements, bothin the manner of fabricating and using the selector described herein, will readily occur to those skilled in the art.

What is claimed is:

1. A touch-sensitive signalling device, which comprises,

a conductive member and spaced apart metallized electret means supported together in spaced juxtaposition in a transducer-like configuration for generating a voltage pulse in response to a tactile displacement of said electret means with relation to said conductive member, the thickness of said electret means and the spacing between said conductive member and said electret means being respectively proportioned to render said configuration insensitive to nontactile displacement forces,

means for directing an externally applied tactile force to said configuration for displacing said electret means with relation to said conductive member, and

means in circuit relation with said conductive member and said electret means for delivering to an external circuit the voltage developed between said conductive member and said electret means in response to a tactile displacement of said electret means.

2. A touch-sensitive signalling device as defined in claim 1, wherein,

said metallized electret means comprises a plurality of thin film electrets, each with a conductive coating on one surface.

3. A touch-sensitive signalling device as defined in claim 1, wherein,

said metallized electret means comprises at least one thin film electret with a conductive coating on one surface thereof, and

an uncharged auxiliary film overlaying said film electret to sandwich said film electret between said auxiliary film and said conductive member. 4. A touch-sensitive signalling device, which comprises, an array of independent conductive members, metallized electret means supported in spaced juxtaposition to said conductive members,

means for restricting the application of an externally applied displacement force to said electret means to locations in substantial alignment with each of said conductive members, and

means in circuit relation with each of said conductive members and said electret means for delivering to an external circuit the voltage developed between a selected conductive member and said metallized electret means in response to a mechanical displacement of said electret in the location of said selected member.

5. A touch-sensitive signalling device as defined in claim 4, wherein,

said metallized electret means comprises a thin film electret with a conductive coating on one surface thereof.

6. A touch-sensitive signalling device as defined in claim 4, wherein,

said metallized electret means comprises a plurality of thin film electrets, each with a conductive coating on one surface, and

means for supporting said film electrets apart from one another by a distance commensurate with the parameters of said film electrets.

7. A touch-sensitive signalling device as defined in claim 4, wherein,

said metallized electret means comprises at least one foil electret and an overlaying uncharged pliable protective layer. 8. A touch-sensitive signalling device as defined in claim 4, wherein,

said means for restricting the application of an externally applied displacement force to said electret means comprises, an apertured cover member overlaying said electret means 10 with said apertures in substantial alignment, respectively, with said conductive members. 9. A touch-sensitive signalling device as defined in claim 4, wherein,

.said means for restricting the application of an externally applied displacement force to said electret means comprises, a plurality of individual pushbuttons in substantial alignment, respectively, with said conductive members. 2o 10. A touch-sensitive selector which comprises, in combination,

an array of conductive members, a single sheet foil electret, comprising a thin film electret metallized with a conductive coating on one surface thereof, supported in spaced juxtaposition to said members with said conductive coating away from said members, an insulating apertured cover member overlaying said foil electret with said apertures in substantial alignment with said conductive members, and

means responsive to the voltage developed between a conductive member and said conductive coating on said electret occasioned by the displacement of that foil electret portion in juxtaposition to said member. 11. A touch-sensitive selector as defined in claim 10,

wherein,

said array of conductive members comprises, an insulating support with recesses at selected locations therein, a plane conductive element embedded in each of said recesses at a slight distance beneath the front surface of said support, and means for connecting said conductive elements indepen-. dently to said voltage responsive means. 12. A touch-sensitive selector as defined in claim 10,

wherein said foil electret is polarized with different polarities at discrete areas substantially aligned with said conductive elements.

13. An electro-mechanical transducer which comprises,

an array of independent conductive elements,

a plurality of thin pliable films, at least one of which com prises a foil electret with a conductive element on one surface thereof, supported in spaced juxtaposition to said independent conductive elements with said conductive elements of all said electrets facing away from said independent elements,

an apertured cover member overlaying said thin films with said apertures in substantial alignment withconductive elements of said array, and

means responsive to a voltage change between selected conductive elements occasioned by the displacement of that foil electret portion in juxtaposition to said array element.

14. An electro-mechanical transducer as defined in claim 13, wherein,

at least two of said thin pliable films are foil electrets held slightly apart from one another, each with their conductive elements facing away from the conductive elements of said array, and

wherein said means responsive to a voltage change includes means for delivering voltage changes occasioned by the individual displacement of each of said foil electrets to an external circuit.

15. An electro-mechanical transducer as defined in claim 75 14,wherein,

said means responsive to a voltage change includes means for altering the time of delivery of said individual voltage changes to said external circuit.

16. An electro-mechanical transducer as defined in claim 14, wherein,

each of said foil electrets is polarized with selectively different polarities at discrete areas substantially in the vicinity of said conductive elements.

17. An electro-mechanical transducer as defined in claim 13, wherein,

at least two of said thin pliable films are foil electrets held slightly apart from one another, each with their conductive coating facing away from the conductive elements of said array, and

wherein said voltage responsive means associated with each of said independent conductive elements includes,

means for delaying the voltage change on selected ones of said independent conductive elements in response to the displacement of selected foil electrets, and

means for combining said delayed voltage changes from selected ones of said elements with the voltage changes from all remaining conductive elements of said array to produce a composite signal.

18. An electro-mechanical transducer as defined in claim 13, wherein,

said conductive element on said surface of said foil electret is restricted to areas substantially aligned with said independent conductive elements.

19. A touch-sensitive signalling device, which comprises,

a plurality of electret films, each with a conductive coating on one surface, and each supported in spaced juxtaposition to one another,

means for restricting the application of an externally applied displacement force to said electret films, and

means in circuit relation with said conductive coatings on said electret films for delivering to an external circuit the voltage developed therebetween in response to a mechanical displacement of at least one of said electret films.

20. A touch-sensitive signalling device as defined in claim 19 wherein all of said electret films are of like polarity.

21. A touch-sensitive signalling device as defined in claim 19, wherein,

selected ones of said electret films are of one polarity and the remaining ones are of the opposite polarity.

22. A touch-sensitive signalling device as defined in claim 19, wherein,

said electret films are polarized with selected polarities at different lateral locations therein.

23. A touch-sensitive signalling device as defined in claim 19, wherein,

said means for delivering said developed voltage changes to an external circuit includes, means for altering the time of delivery of said individually developed voltagechanges.

24. In a touch-sensitive signalling device in which a biased conductive diaphragm is supported in spaced juxtaposition to a conductive member and in which a voltage is developed between said biased conductive diaphragm and said conductive member in response to a mechanical displacement of said diaphragm, that improvement in which,

said conductive diaphragm comprises at least one metallized electret film,

and in which an externally applied mechanical action is directed to a predetermined location for displacing said electret film with relation to said conductive member.

25. A multiple element electrostatic transducer which comprises, in combination,

a planar array of mutually insulated conductive elements,

a foil-electret diaphragm commensurate in area with and held in close proximity to said array of conductive elements, and

means for recovering electric signals individually, from said conductive elements in response to a displacement of said foil-electret portion juxtaposed respectively to said individual ones of said conductive elements.

26. A touch-sensitive signalling device as defined in claim 1 wherein,

said means for directing an externally applied tactile force to said configuration comprises mechanical actuator means as an interface between an applied tactile force and said electret means.

I UNKTED STATES PATENT OFFICE CERlFlQAlE i CCRRECHCN 1 Patent No 3,668., 417 Date' June 6,l 972 Inventofls) Gerhard M. Sessler-Robert L. Wallace a E go-James E. west I L It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, lines 33 and 3 1, that portion of Equations 1) and (2) reading second occurrence, should read Column 3, that portion of Equation l) reading "a(t)" should read -a(t)-=-. 0

Column e 5, that portion of the'forr nula reading should read t Column 4, line 22', "RC t should read --RC t Column 1, line 26, "RC t should read --RC t Signed and sealed this 19th day of November 19%,

(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents g. P I l g Patent No.

Inventor ls) Gerhard M. Sessler-Robert L, Wallace Jra-James Eb West l It is certified that error appears in the above-identified patent: and that said Letters Patent are hereby corrected as shown below;

Column Column Column (SEAL) UNrrEh STATES PATENT OFFICE CERTiFMATE CORREQTIN 3,668 A17 Date June @1272 lines 33 and 3 4, that portion of Equations reading second occurrence, should read that portion of Equation 4) reading read ---a(t)--.

line 22', "RC 0 should read --RC 13 line 26, 2

should read --RC t Signed and sealed this 19th day of November 19 Attest:

MCCOY M. GIBSON JR. Attesting Officer C. MARSHALL DANN Commissioner of Paterts I l I I l I should UNnEo STATES PATENT OFFICE CERTIFICATE CF CORRECTION Patent No. 3,668, 11? Date June 6,1972

Invencoi-(s) Gerhard M. Sessler-Bobert L. Wallace m Jrv-James Em West l It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, lines 33 and 3 that portion of Equations (1) and (2) reading; second occurrence, should read --*-5 Column 3, that portion of Equation-( l) reading "a t)" should read --a(t)--. 0

Column line 6, that portion of thehformula reading t 2 should read t Column 4, line 22', "RC t should read --RC t Column l, line 26, "BC t should read -RC t Signed and sealed this 19th day of November 197 4.

(SEAL) Attest:

McCOY M. GIBSON JR. V c. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. A touch-sensitive signalling device, which comprises, a conductive member and spaced apart metallized electret means supported together in spaced juxtaposition in a transducer-like configuration for generating a voltage pulse in response to a tactile displacement of said electret means with relation to said conductive member, the thickness of said electret means and the spacing between said conductive member and said electret means being respectively proportioned to render said configuration insensitive to nontactile displacement forces, means for directing an externally applied tactile force to said configuration for displacing said electret means with relation to said conductive member, and means in circuit relation with said conductive member and said electret means for delivering to an external circuit the voltage developed between said conductive member and said electret means in response to a tactile displacement of said electret means.
 2. A touch-sensitive signalling device as defined in claim 1, wherein, said metallized electret means comprises a plurality of thin film electrets, each with a conductive coating on one surface.
 3. A touch-sensitive signalling device as defined in claim 1, wherein, said metallized electret means comprises at least one thin film electret with a conductive coating on one surface thereof, and an uncharged auxiliary film overlaying said film electret to sandwich said film electret between said auxiliary film and said conductive member.
 4. A touch-sensitive signalling device, which comprises, an array of independent conductive members, metallized electret means supported in spaced juxtaposition to said conductive members, means for restricting the application of an externally applied displacement force to said electret means to locations in substantial alignment with each of said conductive members, and means in circuit relation with each of said conductive members and said electret means for delivering to an external circuit the voltage developed between a selected conductive member and said metallized electret means in response to a mechanical displacement of said electret in the location of said selected member.
 5. A touch-sensitive signalling device as defined in claim 4, wherein, said metallized electret means comprises a thin film electret with a conductive coating on one surface thereof.
 6. A touch-sensitive signalling device as defined in claim 4, wherein, said metallized electret means comprises a plurality of thin film electrets, each with a conductive coating on one surface, and means for supporting said film electrets apart from one another by a distance commensurate with the parameters of said film electrets.
 7. A touch-sensitive signalling device as defined in claim 4, wherein, said metallized electret means comprises at least one foil electret and an overlaying uncharged pliable protective layer.
 8. A touch-sensitive signalling device as defined in claim 4, wherein, said means for restricting the application of an externally applied displacement force to said electret means comprises, an apertured cover member overlaying said electret means with said apertures in substantial alignment, respectively, with said conductive members.
 9. A touch-sensitive signalling device as defined in claim 4, wherein, said means for restricting the application of an externally applied displacement force to said electret means comprises, a plurality of individual pushbuttons in substantial alignment, respectively, with said conductive members.
 10. A touch-sensitive selector which comprises, in combination, an array of conductive members, a single sheet foil electret, comprising a thin film electret metallized with a conductive coating on one surface thereof, supported in spaced juxtaposition to said members with said conductive coating away from said members, an insulating apertured cover member overlaying said foil electret with said apertures in substantial alignment with said conductive members, and means responsive to the voltage developed between a conductive member and said conductive coating on said electret occasioned by the displacement of that foil electret portion in juxtaposition to said member.
 11. A touch-sensitive selector as defined in claim 10, wherein, said array of conductive members comprises, an insulating support with recesses at selected locations therein, a plane conductive element embedded in each of said recesses at a slight distance beneath the front surface of said support, and means for connecting said conductive elements independently to said voltage responsive means.
 12. A touch-sensitive selector as defined in claim 10, wherein said foil electret is polarized with different polarities at discrete areas substantially aligned with said conductive elements.
 13. An electro-mechanical transducer which comprisEs, an array of independent conductive elements, a plurality of thin pliable films, at least one of which comprises a foil electret with a conductive element on one surface thereof, supported in spaced juxtaposition to said independent conductive elements with said conductive elements of all said electrets facing away from said independent elements, an apertured cover member overlaying said thin films with said apertures in substantial alignment with conductive elements of said array, and means responsive to a voltage change between selected conductive elements occasioned by the displacement of that foil electret portion in juxtaposition to said array element.
 14. An electro-mechanical transducer as defined in claim 13, wherein, at least two of said thin pliable films are foil electrets held slightly apart from one another, each with their conductive elements facing away from the conductive elements of said array, and wherein said means responsive to a voltage change includes means for delivering voltage changes occasioned by the individual displacement of each of said foil electrets to an external circuit.
 15. An electro-mechanical transducer as defined in claim 14, wherein, said means responsive to a voltage change includes means for altering the time of delivery of said individual voltage changes to said external circuit.
 16. An electro-mechanical transducer as defined in claim 14, wherein, each of said foil electrets is polarized with selectively different polarities at discrete areas substantially in the vicinity of said conductive elements.
 17. An electro-mechanical transducer as defined in claim 13, wherein, at least two of said thin pliable films are foil electrets held slightly apart from one another, each with their conductive coating facing away from the conductive elements of said array, and wherein said voltage responsive means associated with each of said independent conductive elements includes, means for delaying the voltage change on selected ones of said independent conductive elements in response to the displacement of selected foil electrets, and means for combining said delayed voltage changes from selected ones of said elements with the voltage changes from all remaining conductive elements of said array to produce a composite signal.
 18. An electro-mechanical transducer as defined in claim 13, wherein, said conductive element on said surface of said foil electret is restricted to areas substantially aligned with said independent conductive elements.
 19. A touch-sensitive signalling device, which comprises, a plurality of electret films, each with a conductive coating on one surface, and each supported in spaced juxtaposition to one another, means for restricting the application of an externally applied displacement force to said electret films, and means in circuit relation with said conductive coatings on said electret films for delivering to an external circuit the voltage developed therebetween in response to a mechanical displacement of at least one of said electret films.
 20. A touch-sensitive signalling device as defined in claim 19 wherein all of said electret films are of like polarity.
 21. A touch-sensitive signalling device as defined in claim 19, wherein, selected ones of said electret films are of one polarity and the remaining ones are of the opposite polarity.
 22. A touch-sensitive signalling device as defined in claim 19, wherein, said electret films are polarized with selected polarities at different lateral locations therein.
 23. A touch-sensitive signalling device as defined in claim 19, wherein, said means for delivering said developed voltage changes to an external circuit includes, means for altering the time of delivery of said individually developed voltage changes.
 24. In a touch-sensitive signalling device in which a biased conductive diaphragm is supported in spaced juxtaposition to a conductive membeR and in which a voltage is developed between said biased conductive diaphragm and said conductive member in response to a mechanical displacement of said diaphragm, that improvement in which, said conductive diaphragm comprises at least one metallized electret film, and in which an externally applied mechanical action is directed to a predetermined location for displacing said electret film with relation to said conductive member.
 25. A multiple element electrostatic transducer which comprises, in combination, a planar array of mutually insulated conductive elements, a foil-electret diaphragm commensurate in area with and held in close proximity to said array of conductive elements, and means for recovering electric signals individually from said conductive elements in response to a displacement of said foil-electret portion juxtaposed respectively to said individual ones of said conductive elements.
 26. A touch-sensitive signalling device as defined in claim 1, wherein, said means for directing an externally applied tactile force to said configuration comprises mechanical actuator means as an interface between an applied tactile force and said electret means. 